Steer-By-Wire Control System Adem Kader Swarthmore College Department of Engineering Advisor: Erik Cheever May 2006
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Steer-by-Wire Control System Adem Kader Swarthmore College Department of Engineering Advisor: Erik Cheever May 2006 Abstract The automotive industry has already implemented many advanced computer systems in an attempt to increase safety and comfort of drivers. In parallel with these advancements we see a big shift from mechanical systems to electrical systems and steer-by-wire is another implementation that is very promising in terms of safety and functionality. Already, there are some commercial prototypes of such ‘by-wire’ systems[1] and there is a lot of research, both academic[2] and comercial[3], in the field. For my Engineering Senior Design Projet at Swarthmore College, I chose to work on a steer-by-wire system to gain more insight into control theory and I thought the double-control system that provided the crucial feedback to the driver was an interesting engineering problem. 1 Contents 1 Introduction 3 1.1 Definition and Benefits of Steer-By-Wire . 3 1.2 Introduction to the Project . 3 2 Design 4 2.1 System Overview . 4 2.2 Physical Components . 4 2.2.1 Steering Actuator . 4 2.2.2 Feedback Motor . 6 2.2.3 Angular Sensors . 6 2.3 Electronics . 7 2.3.1 Microcontrollers . 7 2.3.2 Pulse Width Modulation and the H-Bridge . 9 2.3.3 Motor Controller for Steering Actuator . 10 2.4 Computer Modeling . 10 2.4.1 Simulink Modeling . 10 3 Testing 11 3.1 Testing Procedure . 11 3.2 Determining PID constants . 12 4 Discussion 14 5 Future Work 14 6 Acknowledgements 14 7 APPENDIX 16 2 1 Introduction 1.1 Definition and Benefits of Steer-By-Wire A steer-by-wire system aims to eliminate the physical connection between the steering wheel and the wheels of a car by using electrically controlled motors to change the direction of the wheels and to provide feedback to the driver. Today’s automobiles benefit more and more from the many uses of electronic systems. The integration of a steer-by-wire system can enhance these systems in many ways. In particular, the handling and the safety of the cars can be improved significantly. Since a steer-by-wire system is easily modifiable, different drivers will be able to adjust the system to accommodate their styles and this will enhance handling. I addition, disabled people and the elderly will benefit immensely from steer-by-wire because they will be able to situate the steering wheel to meet special needs. Traction control systems are very closely tied with driving safety and they can be enhanced with steer-by-wire vastly. For instance, in a situation where the car starts oversteering (when the rear of the vehicle heads towards the outside of the corner), the natural instinct of many inexperienced drivers is to turn the steering wheel towards the inside, which in turn causes more oversteer. A steer-by-wire system could be modified to take control in a situation like this to steer to the outside. Since there are virtually no physical connections between the steering wheel and the wheels, a steer-by-wire system can be implemented on different cars easily. The steering wheel could be placed on either side of a car (or anywhere else). Both of these improvements would reduce costs of production and allow a wider range of designs. The downsides of a steer-by-wire system are maintenance and power cost. Concievably steer- by-wire will use more power than the currently used system, however considering the power con- sumption of power steering the power cost will be insignificant. There might also be more electrical failures, but presumably steer-by-wire systems will last longer because they have fewer mechanical parts and will improve safety and therefore help the overall maintenance costs. 1.2 Introduction to the Project I chose to work on a steer-by-wire system control because it is an interesting double-control problem. As seen in Figure 1, my system has two rotary encoders that provide angular information to a computer controller through two microcontrollers. Then the computer controller regulates two external controllers that are necessary for voltage and current adustments. The adjusted output drives the feedback actuator and the steering actuator to their desired positions. Whenever there is a difference in the angles of the two motors, the system tries to bring the difference back to zero, which is the equilibrium and the desired confiuration. The system has two different motors with different parameters that depend on each other and this necessitates two controllers that have to be merged into one big controller that is capable of reacting to outside disturbances. The outside disturbances are the driver and the road. The driver controls the smaller of the two motors in the system since it is easier to steer a motor with lower torque. The road might cause quick disturbances (for instance when a car hits a curb and the wheels instantly change direction) and clearly these are unwanted and dangerous disturbances so a motor with high torque is required to control the wheels in order to minimize destructive interferences. 3 Figure 1: Overview of system 2 Design 2.1 System Overview The steer-by-wire system consists of two main parts. The steering section consists of the steering wheel, the feedback actuator and the feedback actuator angle sensor. The wheel section contains the wheels, the rack and pinion, a steering actuator and the pinion angle sensor. Figure 2 shows the system components. In my system I only demonstrated the double control mechanism and did not implement it in a rack and pinion configuration. The feedback angle sensor provides the steering actuator with its primary input signal and the pinion angle sensor provides the feedback motor primary signal. The small size of the feedback motor lets the driver rotate the steering wheel with little difficulty. As soon as the driver starts steering, the control mechanism tries to push the steering wheel back into place (and the wheels into the position dictated by the current position of the steering wheel) and this mimics the resistive force of a real steering wheel. However, changing the proportional constant of the feedback motor can make it harder/easier for the driver to steer and allows for adjustable steering (with some drawbacks such as more vibration). 2.2 Physical Components 2.2.1 Steering Actuator The steering actuator needs to be very powerful in order to turn the wheels of a car when the car is loaded. Minimizing the effects of unwanted disturbances also requires a powerful motor. In the design, I wanted to use a small wagon as a model and measurements showed that a motor with a torque of about 80lb-in was necessary. Ideally this motor would be a brushless DC motor in order to reduce noise and maximize motor life. However, the high cost and low availability of brushless motors led me to acquire the DC motor shown in Figure 3. This is a Groschopp 50757, a 88.9 lb-in 4 Figure 2: Components of a steer-by-wire system motor that uses a 12V source and draws approximately 3.4 amps. Initially, I wanted to use a small wagon to implement my project and show that I could steer the wagon with a person seated on it. In order to find the right size motor, I put a 200lb weight located in the center of the wagon body and used a 39.5 inch crank to turn the rack of the wagon. With the wheels on a painted floor the force required was about 1.9lbs (and it never exceeded 2lbs). Thus the total torque was measured to be 39.5 · 2 ≈ 80lbs. Although this motor has sufficient torque, the maximum rotation speed is limited to 13.3 RPM. Using only 13.3 RPM, it is impossible to simulate rapid movements experienced in a car, therefore this motor requires gears to increase rotational speed at the expense of reducing force. Figure 3: Steering Actuator The high current characteristics of this motor make it impossible to control using an H-bridge 5 configuration and require a special controller. The specification sheet for this motor can be found in the appendix. 2.2.2 Feedback Motor Figure 4: Feedback Motor The feedback motor does not have to be as powerful as the steering actuator. In fact, it has to be much less powerful in order to be turned easily by a driver. I opted to use the motor seen in Figure 4. It was a used motor I acquired from the department. Initially I tested this motor with full power, and the torque it provided was similar to the torque felt in a real car and I concluded that this would be a sufficient motor. The power requirements were also reasonable. It operates at 12V and draws less then 1.5A, so the low current requirements allow for an H-Bridge controller. 2.2.3 Angular Sensors The angular sensors of the system are very crucial and they need to be very accurate because little perturbations or errors ultimately make the control of the system much harder for a driver. In a real implementation of a steer-by-wire system then would have be very high sensitivity and accuracy in order to minimize risks. In my project I used two optical digital encoders that were used in a previous project. These are BEI Duncan’s EX-11 and MX-15 encoders and both of the sensors are seen in Figure 5. Figure 5: Angular sensors: Incremental encoders 6 Optical digital encoders’ precision and accuracy make them preferable over potentiometers.